Some Ideas

1.have loot bonus effect gathering
2.have xp bonus effect gathering and crafting xp
3.possibly a npc where you can trade in quest items for a token that stacks and you can swap with quest items you need(so quest items don't take up half your bank)
4.a item that hold emeralds example while loot running instead of haveing emeralds just sit in your inv
have an item that can hold emerald and have diff tears like one that can hold 128,256,512.....

 

loot bonus and xp bonus affecting gathering would kind of mess with the balancing of those 2, they're meant as combat IDs and when you're gathering you're not usually fighting too much. this could lead to people making the best loot/xp bonus builds possible, finding a safe gathering spot, and getting way more than intended from gathering.

for quest items, I don't think there's even 18 you'll have at a time. surely you can dedicate 3 rows of your bank to some quest items

 

ya only 18 but if you have muitple classes it can get messy
with the xp and loot bonus i can see that